Asexual Ants Have Sex

Some populations of ants long thought to be asexual get a dose of genetic diversity by having sex.

By Tia Ghose | July 18, 2011

M. SmithiiALEX WILD

Birds do it. Bees do it. But until now, no one thought the fungus farming ant did it. Documenting genetic variations and the presence of sperm, however, researchers have found evidence that some populations of Mycocepurus smithii actually do have sex, according to a study publishing today (July 18) in PNAS.

The study is “a tour de force of both field work and lab work,” said University of California, Berkeley, evolutionary biologist Neil Tsutsui, who was not involved in the study. The finding may offer insight into a long standing question in evolutionary biology about what forces cause species to choose sex over asexual reproduction and vice versa, he said.

On paper, asexuality seems like a winning strategy. Sexless creatures pass on all their genes—as opposed to just half—and “you don’t have to spend huge amounts of energy going around and finding a mate and going through courtship and exposing yourself to disease,” Tsutsui said.

Yet species overwhelmingly engage in the messy, costly business of sex because it provides the genetic variation species need to react quickly to environment and ecosystem changes, said Ulrich Mueller, an integrative biologist at the University of Texas at Austin and a co-author of the study.

But the fungus-farming ant is one of the few species that appeared to adopt a purely asexual lifestyle: researchers had never seen a male in the wild, and ants in the lab produced clonal offspring.

But when Mueller and his colleagues analyzed genetic markers in 35 populations across Central and South America, they found four Amazonian populations in which queens had a different genetic makeup than their offspring, indicating that those offspring were not simply the result of asexual cloning. By contrast, worker ants in the other 31 other colonies were all genetically identical to their queens.

When the researchers dissected the seemingly sexual queens, they found storage organs filled with sperm, a sign that the ants mated at least once. Genetic markers in the sperm matched those in worker ants, but absent in queens, confirming that those workers were produced sexually, though as of yet, no one has seen the elusive fathers. Ants from the other locations also had sperm storage organs, but they were empty, suggesting that they once had the capacity to reproduce sexually but no longer do.

Tracking differences in other, slowly mutating genes to retrace the evolutionary history of the ants, the team confirmed that the common ancestor of the group probably reproduced sexually, and that asexuality had evolved multiple times independently. One advantage of asexuality for colonizing ants is that just one individual, rather than two, can clone itself and establish a new population, Mueller said. But while asexuality may have helped certain M. smithii populations expand their range, those that have sex are probably more resilient to environmental change, Mueller said.

“If you come back in 5 million or 10 million years, there's a good chance the asexual lineages go extinct, but the sexual lineages are still existing.”